This invention relates to a process and system for improving the ability of a cogeneration facility to meet changing power and heat demands, and more particularly to the use of a thermal boost system in combination with a cogeneration facility to achieve control over varying power and process steam demands.
Many large industrial plants generate both their power and heat requirements on site. In a typical system, fuel is burned in boilers to produce steam which is then expanded through one or more turbines to create shaft work. This shaft work can then be used to generate electricity to meet the power needs of the facility. One or more portions of steam are extracted from the turbines at desired pressures to meet the heat and process steam requirements of the facility.
The turbines in such facilities are designed and built to meet specific power and heat requirements, and, when there is a variation in those requirements, significant efficiencies of operation are lost. For example, when the need for steam in a plant is reduced through energy conservation measures or because of a cyclic reduction in demand for process steam, it is often preferred to reduce the amount of steam generated at the boilers to match.
The resulting reduced steam flow through the turbines, however, reduces the power generated and requires the purchase of electrical power from an external source such as a local utility. If this is undesirable because of high costs or unavailability, then steam flow must be maintained to meet power needs. The excess steam can be vented although this results in a waste of the cost of the fuel needed to generate it. Alternatively, the excess steam can be further expanded through middle and/or low pressure turbines to produce more shaft work for power generation. However, further expansion may cause wetness problems and excessive wear on turbine blades requiring the separation and extraction of liquid. Moreover, design and fabrication of the turbine becomes progressively more difficult because of the low density of the steam at lower pressures. Neither of the above alternatives has proven to be satisfactory for cogeneration applications.
Accordingly, the need exists in the art for a process and system which allows for more efficient and flexible utilization and control of the power and steam produced by a cogeneration facility in response to varying power and process steam demands.